This invention relates to test structure patterns used in semiconductor manufacturing, and in particular to test structure patterns used to determine the field-to-field alignment of a stepper in a lithographic process.
Photomasks are an integral component in the lithographic process of semiconductor manufacturing. Semiconductor manufacturers use photomasks to optically transfer (e.g., print) images of devices (e.g., integrated circuits) onto semiconductor wafers. A lithography tool called stepper projects light through the photomask to print the image of one or more devices onto a field on a silicon wafer coated with photoresist. The stepper then moves (e.g., steps) the wafer and the image is exposed once again onto another field on the wafer. This process is repeated for the entire wafer surface. When using a positive photoresist, the exposed portions of the photoresist are removed so areas of the wafer underneath can either be etched to form channels or be deposited with other materials. This process can be reversed using a negative photoresist where the unexposed portions of the photoresist are removed.
FIG. 1 illustrates a path 102 of a stepper on a wafer 100 coated with photoresist. The stepper prints the image of one or more devices on fields 200-1, 200-2 . . . 200-j . . . 200-m on wafer 100, where xe2x80x9cjxe2x80x9d and xe2x80x9cmxe2x80x9d are variables.
FIG. 2 illustrates that each field partially overlaps neighboring fields in scribe lanes (also called xe2x80x9cscribe streetsxe2x80x9d or xe2x80x9cscribe linesxe2x80x9d) where a dicing tool cuts to separate the fields. For example, the left edge of field 200-1 and the right edge of field 200-2 overlap in scribe lanes 202 and 210, the lower edge of field 200-1 and the upper edge of field 200-7 overlap in scribe lanes 208 and 210, and the lower left corner of field 200-1 and the upper right corner of field 200-6 overlap in scribe lane 210. Similarly, the upper edge of field 200-6 and the lower edge of field 200-2 overlap in scribe lanes 204 and 210, and the right edge of field 200-6 and the left edge of field 200-7 overlap in scribe lanes 206 and 210.
In lithography, field-to-field alignment of the stepper is critical because it impacts all future masking alignments, wafer sort, and ultimately the assembly process. If the field alignment is poor, it directly impacts sort yield and assembly yield. The assembly process can be halted if the field-to-field alignment is so poor that the dicing tool cuts into the production die and damages the die and itself. By quantifying the amount of misalignment, steppers that need maintenance may be detected before they damage or destroy product wafers.
Thus, what is needed is a production friendly, field-to-field alignment tool that allows the ability to rapidly and accurately measure and quantify the field-to-field alignment.
In one embodiment of the invention, a test structure pattern includes a first comb having a first set of tines, and a second comb having a second set of tines of the same width and spacing as the first set of tines. When the test structure pattern is stepped between fields on a wafer, the first comb and the second comb at least partially overlap on photoresist over a scribe lane between the fields. When the photoresist is developed, the overlap of the first comb and the second comb generates a metal comb. Electrical continuity is checked for the metal tines of the metal comb to determine the amount of misalignment of the fields.